There are already several great threads explaining 2F/60 Series Aisin carbs and what the secondary does, I’ll link those in the second post. So what’s the point of this thread? To discuss tuning the secondary to make it more useful. I’m just going through this and some of it is me thinking out loud, so feel free to argue with me.
After noticing my secondary wasn’t working, I set about to fix it. I rebuilt this carb a year ago and found two issues: the diaphragm was installed incorrectly, and the vacuum passage to the secondary diaphragm was plugged with gunk. There’s some cause and effect there. Oops … my bad.
I’ve had several carbs that work fine on my 60s and noticed that even when the secondary is working perfectly, it rarely opens. I live in Denver and the secondary will come into play driving up long highway grades in the mountains, that’s really it. An effect of lower air density at altitude? Not sure. Even the carbs I’ve gotten from low altitude places have squeaky clean secondaries though. In all other circumstances I’m always having to shift gears before the secondary can build enough vacuum to fully open.
A couple of facts about the secondary, carbs, and engines (see the linked posts below for deeper dives):
-the primary linkages mechanically crack open the secondary a very small amount; this enables the next step
-the secondary then fully opens via a vacuum operated diaphragm & linkage; the vacuum comes from inside the venturi, there’s a passage near the secondary butterfly that sees a vacuum when the butterfly is cracked open
-the engine is an air pump, the carb is an imposed restriction so you can control the air pump
-the carburetor deals with flow of air and adding fuel to that flow; the secondary opening fully based on vacuum has to do with that flow; power is, among other things, also related to this flow. More flow = more power (obviously there’s more to it, but this is what I’m focusing on)
-other things control that flow like intake & exhaust valve size & timing, head design, and displacement, but that’s beyond the scope here
So how can I shift the secondary’s contribution to air flow down into the usable range a little more? How can I do that without sacrificing the secondary’s contribution higher up? I think it’s a balance! I use the words down and up generally to reference both rpm and engine load. We can start by looking at the carb rebuild & setup procedures in the FSM. There are three key things here: how quickly does the secondary crack open, how much does it crack open, and - not mentioned in the FSM - how quickly does the diaphragm make it open fully?
1. The initial cracking open of the secondary happens mechanically by action of levers connected to the primary, and ultimately connected to the pedal. In the photo below the circled tab controls how soon the secondary starts to crack open. Bent way down as shown will start the secondary moving sooner than what’s specified in the FSM.
2. How much the secondary gets cracked open is controlled by the lever circled below. There is an arm attached to the primary that contacts that lever. Bending the circled lever upwards will make the secondary crack open more, but be careful: if you go too far the butterfly will be open at idle. This is not good and tuning your carb to make the engine idle nicely will be difficult or impossible. I went right up to that point, stopping my adjustment just before the secondary was open at rest. In total it now opens about 4-5mm. The spec in the FSM is an angle of the secondary and it equates to about 1-1.5mm of space between the venturi wall and the butterfly. So now there’s more air flow mechanically, under your pedal foot.
A couple notes here … the shape of the arm attached to the primary - the one that contacts the lever circled above - is rounded and kind of horizontal to the axis of rotation. This limits the total amount it can effect the circled lever. If it were shaped differently, it could crack open the secondary more. Also, because the circled lever is so short (left to right), the arm that contacts it can partially slip off the right side, causing the secondary to go just a smidge more closed. This wouldn’t happen if the lever were longer. Out of all the carbs I’ve had on my two 60s it always feels like it makes the most power when the pedal is just a hair back from hitting the floor - I wonder if this is why. It’s a very, very slight feeling.
3. Last is how quickly the diaphragm pulls the secondary fully open. I haven’t gotten into this yet but I think there are a couple ways to adjust it. Inside the diaphragm is a spring, and the vacuum has to pull against the spring pressure. The hot rod carb companies like Holley and Edelbrock make spring kits for this - you get a package of 5-6 and do trial and error until you find one that works for the goal. I have a set of those on the way. Second is the small orifice in the casting that holds the diaphragm. It’s there to restrict vacuum flow. I wonder how boring out the orifice would change things. My initial thought is that the orifice controls how quick or how “snappy” the diaphragms actuation is. I think both of these need caution: if the secondary opens too fast you’d probably get a momentary bog as air rushes in through the carb and fuel isn’t quite being pulled into the venturi yet. It would be a split second thing, but a stumble nonetheless. Probably why some hot rod carbs are “double pumpers” and have an accelerator pump for the secondaries. I’m going to go lighter on the spring pressure until it stumbles, and then go back one spring heavier. If the diaphragm snaps the secondary shut too quickly, you’d probably feel the truck lurch a little bit when you relaxed on the pedal, too. Not good. I’ll leave the orifice for now but might come back to it. Here’s the orifice/restriction below, it’s a brass piece pressed into the vacuum passage in the diaphragm housing:
So by adjusting the lever arms in #1 & #2 I now feel a little more juice around 1800 rpm, and I can feel a little more above 2800 as well. This is a 2F, so it isn’t much - but there’s a little something more that I can feel. Prior to this there wasn’t much beyond 2800rpm or so, unless I was climbing something like Vail Pass. Even then it wasn’t enough extra to keep me from losing speed for a good length of time (as the secondary is probably pretty slow getting to full open). That is a definite downward shift (rpm and load) in the secondary putting in the work. What I don’t want to do is have the secondary fully come into play too early and have nothing left to give in a Vail Pass situation, or needlessly chew through fuel driving around town. So far this has made a decent amount of difference getting onto the highway, but I haven’t done a big climb since yet so that’s TBD.
Thanks to @mattressking for letting me bounce ideas around and @RDub for the same, for half-assed going through Bournouli’s equation with me, and for generally geekery as usual.
Alright, time for y’all to tear apart my ideas!
After noticing my secondary wasn’t working, I set about to fix it. I rebuilt this carb a year ago and found two issues: the diaphragm was installed incorrectly, and the vacuum passage to the secondary diaphragm was plugged with gunk. There’s some cause and effect there. Oops … my bad.
I’ve had several carbs that work fine on my 60s and noticed that even when the secondary is working perfectly, it rarely opens. I live in Denver and the secondary will come into play driving up long highway grades in the mountains, that’s really it. An effect of lower air density at altitude? Not sure. Even the carbs I’ve gotten from low altitude places have squeaky clean secondaries though. In all other circumstances I’m always having to shift gears before the secondary can build enough vacuum to fully open.
A couple of facts about the secondary, carbs, and engines (see the linked posts below for deeper dives):
-the primary linkages mechanically crack open the secondary a very small amount; this enables the next step
-the secondary then fully opens via a vacuum operated diaphragm & linkage; the vacuum comes from inside the venturi, there’s a passage near the secondary butterfly that sees a vacuum when the butterfly is cracked open
-the engine is an air pump, the carb is an imposed restriction so you can control the air pump
-the carburetor deals with flow of air and adding fuel to that flow; the secondary opening fully based on vacuum has to do with that flow; power is, among other things, also related to this flow. More flow = more power (obviously there’s more to it, but this is what I’m focusing on)
-other things control that flow like intake & exhaust valve size & timing, head design, and displacement, but that’s beyond the scope here
So how can I shift the secondary’s contribution to air flow down into the usable range a little more? How can I do that without sacrificing the secondary’s contribution higher up? I think it’s a balance! I use the words down and up generally to reference both rpm and engine load. We can start by looking at the carb rebuild & setup procedures in the FSM. There are three key things here: how quickly does the secondary crack open, how much does it crack open, and - not mentioned in the FSM - how quickly does the diaphragm make it open fully?
1. The initial cracking open of the secondary happens mechanically by action of levers connected to the primary, and ultimately connected to the pedal. In the photo below the circled tab controls how soon the secondary starts to crack open. Bent way down as shown will start the secondary moving sooner than what’s specified in the FSM.
2. How much the secondary gets cracked open is controlled by the lever circled below. There is an arm attached to the primary that contacts that lever. Bending the circled lever upwards will make the secondary crack open more, but be careful: if you go too far the butterfly will be open at idle. This is not good and tuning your carb to make the engine idle nicely will be difficult or impossible. I went right up to that point, stopping my adjustment just before the secondary was open at rest. In total it now opens about 4-5mm. The spec in the FSM is an angle of the secondary and it equates to about 1-1.5mm of space between the venturi wall and the butterfly. So now there’s more air flow mechanically, under your pedal foot.
A couple notes here … the shape of the arm attached to the primary - the one that contacts the lever circled above - is rounded and kind of horizontal to the axis of rotation. This limits the total amount it can effect the circled lever. If it were shaped differently, it could crack open the secondary more. Also, because the circled lever is so short (left to right), the arm that contacts it can partially slip off the right side, causing the secondary to go just a smidge more closed. This wouldn’t happen if the lever were longer. Out of all the carbs I’ve had on my two 60s it always feels like it makes the most power when the pedal is just a hair back from hitting the floor - I wonder if this is why. It’s a very, very slight feeling.
3. Last is how quickly the diaphragm pulls the secondary fully open. I haven’t gotten into this yet but I think there are a couple ways to adjust it. Inside the diaphragm is a spring, and the vacuum has to pull against the spring pressure. The hot rod carb companies like Holley and Edelbrock make spring kits for this - you get a package of 5-6 and do trial and error until you find one that works for the goal. I have a set of those on the way. Second is the small orifice in the casting that holds the diaphragm. It’s there to restrict vacuum flow. I wonder how boring out the orifice would change things. My initial thought is that the orifice controls how quick or how “snappy” the diaphragms actuation is. I think both of these need caution: if the secondary opens too fast you’d probably get a momentary bog as air rushes in through the carb and fuel isn’t quite being pulled into the venturi yet. It would be a split second thing, but a stumble nonetheless. Probably why some hot rod carbs are “double pumpers” and have an accelerator pump for the secondaries. I’m going to go lighter on the spring pressure until it stumbles, and then go back one spring heavier. If the diaphragm snaps the secondary shut too quickly, you’d probably feel the truck lurch a little bit when you relaxed on the pedal, too. Not good. I’ll leave the orifice for now but might come back to it. Here’s the orifice/restriction below, it’s a brass piece pressed into the vacuum passage in the diaphragm housing:
So by adjusting the lever arms in #1 & #2 I now feel a little more juice around 1800 rpm, and I can feel a little more above 2800 as well. This is a 2F, so it isn’t much - but there’s a little something more that I can feel. Prior to this there wasn’t much beyond 2800rpm or so, unless I was climbing something like Vail Pass. Even then it wasn’t enough extra to keep me from losing speed for a good length of time (as the secondary is probably pretty slow getting to full open). That is a definite downward shift (rpm and load) in the secondary putting in the work. What I don’t want to do is have the secondary fully come into play too early and have nothing left to give in a Vail Pass situation, or needlessly chew through fuel driving around town. So far this has made a decent amount of difference getting onto the highway, but I haven’t done a big climb since yet so that’s TBD.
Thanks to @mattressking for letting me bounce ideas around and @RDub for the same, for half-assed going through Bournouli’s equation with me, and for generally geekery as usual.
Alright, time for y’all to tear apart my ideas!
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